1. Technical Field
This invention relates to methods and apparatus for performing track searches across optical recording discs. It is particularly useful in connection with track searches across read/write type optical discs.
2. Background of the Invention
It is sometimes desired to jump from one track to a different track on an optical recording disc such as a compact disc. This is accomplished by moving the optical pickup head rapidly across the compact disc in a radial direction and counting the disc tracks as they are crossed. When the proper number of tracks have been crossed, the search is terminated and normal operations are resumed. During the rapid radial search movement, the optical pickup head generates a pair of track crossing signals which are used for track counting purposes and for determining the track crossing direction. One of these signals is an in-phase signal and is commonly referred to as a TX signal. The other is a quadrature-phase signal which is commonly referred to as an RX signal. These are pulse type signals of the same frequency which are offset in time relative to one another. More particularly, one signal leads the other by a factor of 90xc2x0. The search direction is determined by which signal is leading.
A problem is encountered when performing track searches across read/write type optical discs. The problem arises when trying to count track crossings across blank (unrecorded) areas on the read/write disc. During this time, the quadrature-phase signal or RX signal is lost due to the low contrast of reflected light off the unwritten tracks. In blank (unwritten) areas, the RX signal level becomes very small and unreadable. This loss of the RX signal causes the track count to be lost because the direction information cannot be determined by TX alone.
This invention detects when the optical pickup head is in a blank or unrecorded area of a read/write disc. An artificially-generated RX signal is derived from the TX signal and is used as a substitute when the track search is in a blank region of the disc. This substitute RX or xe2x80x9cfake RXxe2x80x9d signal allows the track counting to continue uninterrupted after crossing the blank boundary, making the track jumps xe2x80x9cseamlessxe2x80x9d. The key to this invention is the rapid detection of the blank area within one-half of a track crossing and the fast insertion of a substitute signal for the RX signal that is lost as the optical pickup unit crosses the boundary from recorded tracks to blank tracks.
For a better understanding of the present invention, together with other and further advantages and features thereof, reference is made to the following description taken in connection with the accompanying drawings, the scope of the invention being pointed out in the appended claims.